Fully assembled, she stands 42.3 feet tall and 428.5 feet long. “Harriet,” the name given to the German-made tunnel boring machine by the Miami-Dade County Girl Scouts Troop, is currently digging her way from Watson Island to Dodge Island in Florida, a 4,200 foot trek. Two twin tunnels are being constructed with plans to include two lanes of traffic, curbs and walkways within each respective tunnel.
The behemoth machine works 20 hours a day, stopping for 4 hours only for maintenance. Harriet, whose name comes from Harriet Tubman, a woman famous for her work as a conductor for the Underground Railroad, makes slow but steady progress on her subterranean journey by spinning as a cutting wheel, boring a hole 39 feet in diameter.
Perhaps one of the machine’s most interesting capabilities is its ability to erect precast concrete segments as it progresses underground. These segments are the finished walls of the tunnel and, save for a bit of grout pumped in to seal the tunnel, require no further effort on the part of construction crews to prepare the tunnel walls. The machine-wonder cost approximately 45 million dollars and holds 12-16 workers inside, also allowing 12-14 to work on top of it. Since excavation began in November of 2011, Harriet is on schedule to finish the first half of the mega project sometime this month.
Upon reaching Dodge Island, Harriet will have to be partially disassembled, turned around, and reassembled for the journey back to construct the second of the twin tunnels. The tunnels themselves should do much to ease traffic in highly congested areas, and also provide an alternate route into a city hub.